To cite package PK in publications use:

Jaki T, Wolfsegger M (2011). “Estimation of pharmacokinetic parameters with the R package PK.” Pharmaceutical statistics, 10(3), 284-288. DOI: 10.1002/pst.449, https://CRAN.R-project.org/package=PK.

Corresponding BibTeX entry:

  @Article{,
    title = {Estimation of pharmacokinetic parameters with the R
      package PK},
    author = {Thomas Jaki and Martin Wolfsegger},
    year = {2011},
    journal = {Pharmaceutical statistics},
    volume = {10},
    number = {3},
    pages = {284-288},
    note = {DOI: 10.1002/pst.449},
    url = {https://CRAN.R-project.org/package=PK},
  }